Human biometrics using static body shape is a re-emerging technique to uniquely identify people. Over the last 15 years, numerous 3D body scanners have been developed that can digitize the human form with a high degree of accuracy. These scanners are commercially available and utilize optical or radar frequencies to form digital images of humans for body measurement applications. Up to this point, commercial applications for these scanners have focused on the apparel industry and national size surveys. There are three different categories of body scanners that include laser-scanning, white light projection and radar. Radar is the only one of these scanner technologies that can penetrate clothing, hair and optically opaque plastic disguises.
Holographic radar imaging technology uses harmless microwaves or millimeter waves to illuminate a person under surveillance. These waves readily penetrate through clothing and reflect off water in the skin. The reflected signals are digitized and sent to high-speed computers to form three-dimensional (3-D) images of the person and any concealed objects hidden in their clothes. Microwaves and millimeter waves are electromagnetic waves in the 3-30 GHz and 30-300 GHz ranges respectively. These waves have wavelengths ranging from 1-10 cm for microwaves and 0.1-1.0 cm for millimeter-waves. High resolution imaging in these frequency ranges requires fundamentally different techniques than are used in the optical and infrared ranges. In particular, large apertures with sizes comparable to the target are required to obtain high-quality images. Data collection may be performed by mechanically scanning a measurement system comprising a linear transceiver array using a cylindrical scanner. 3D images are formed by using a combined cylindrical holographic radar imaging technique. The cylindrical data are divided into a number of arc segments, each of which is reconstructed separately and incoherently summed with the others. The resulting 3-D combined image can then be viewed at any desired angle using digital rendering techniques. Computer reconstruction requirements are reduced by performing the minimum number of reconstructions. Each angular segment is typically 90 degrees in extent. The microwave/millimeter-wave linear transceiver array emits a diverging beam that interacts with the imaging target and then measures the amplitude and phase of the scattered wavefronts at each sampled position and frequency over the cylindrical aperture. This dataset is three-dimensional with dimensions consisting of the two aperture dimensions and the frequency dimension. Wavefront reconstruction techniques can be used to mathematically focus these data using computer-based image reconstruction algorithms.
Measurements can be performed at distinct radio subbands for processing the data to arrive at an inverted image of the body.